Downhole running cables are used in the oil and gas industry for deploying and retrieving well intervention and logging equipment in a well. For example, tools can be deployed downhole using a slickline spooled out from a drum and guided over sheaves before entering the well. Steel wires are generally chosen for such services to meet the rigorous physical requirements of the service while maintaining tensile strength without sustaining damage. However, if the deployed tool relies on electrical signals, steel wires are not typically used to for communicating the electrical signals. Instead, copper conductors are used for this purposes. Since the copper cannot sustain load, the cable is reinforced with steel wire.
FIG. 1 illustrates a typical rig up system 10 for running cable 12 downhole for various purposes. The system 10 is shown for slickline, but the rig up for wireline (e-line), braided line, Heavy Duty Wireline Fishing (HDWF) line, and the like may use the same general configuration. The difference is that wireline (e-line), braided line, and HDWF line use a grease injection system to maintain well pressure. As shown, slickline instead uses a stuffing box 32.
The cable 12 (e.g., slickline, braided wireline, electric line, etc.) passes from a drum 22 in a deployment unit 20 to a hay pulley 28, which directs the cable 12 to the sheave on the stuffing box 32. The cable 12 enters the stuffing box 32, passes through a chemical injection sub 34 and a lubricator 36, and passes to a secondary barrier 38 or blow out preventer. Eventually, the cable 12 passes to the Christmas tree 40 through the swab and master valves 42, and then to the well for its intended purposes. Various other components are used with the system as well, but are not described here. When the cable 12 is used for intervention, for example, the rig up system 10 may include cable cutter subs, a tool trap, a tool catcher, check valves, etc.
The stuffing box 32 packs off around the cable 12. The chemical injection sub 34 applies various agents and corrosion inhibitors to the cable 12 during operations. The lubricator 36 is used for inserting and retrieving a tool string (not shown) when the well is under pressure. The secondary barrier 38 can use ram seals to close off around the cable 12 in the event of an emergency or essential maintenance.
For those cables 12 with a smooth outer surface, the stuffing box 32 can use elastomeric seals. Otherwise, grease-injected sealing hardware is used with served or braided cable surfaces. Where a stuffing box 32 cannot be used, for example, a grease injection control head (not shown) can create a seal around the moving cable 12 by injecting grease so the cable 12 can be run for intervention operations in wells under pressure.
The rig up's deployment unit 20 can be skid mounted on the rig or can be part of a deployment truck. The unit 20 stores the cable 12 on the drum 24 that feeds the cable 12 on and off of the unit 20. A winch for the drum 24 has a hydraulic drive powered by a diesel engine or electric power pack that drives the drum 24 to feed or pull the cable 12. The unit 20 may also include depth and tension systems. For example, a weight indicator sensor 29 can be used to measure line tension on the cable 12, and a depth counter 26 can be used to measure the length of cable 12.
As an example, FIG. 2 schematically shows a measuring device 50 that can measure depth and tension for the deployment unit (20). This measuring device 50 uses two wheels 52, 54 to measure depth of the cable 12. The device 50 mounts in front of the drum (24) on a spooling mechanism and can ride back and forth on linear bearings. The cable 12 from the drum (24) completely wraps around both wheels 52, 54 and extends from the wheels 52, 54 to other components to go to the well. Spooling rollers 55, guide rollers 56, and a pressure wheel 58 keep the cable 12 in the wheels 52, 54 and assist in the spooling of the cable 12. Line tension is measured from a load pin axle 53 for the tension wheel 52. A hydraulic load cell may also be included that measures cable tension independently of the electronic load pin. Depth information is provided by an encoder 57 to the unit's control panel (22), which accounts for the size and stretch of the cable.
The cable 12 can come in various arrangements and geometries. Some forms of downhole running cables, such as wirelines, e-lines, braided lines, etc., have wires or strands. During use, the cables 12 are subject to elastic elongation, permanent stretch, breakage, and the like based on the loads, twists, bends, and other actions subjected to the cable. For example, a swivel connector in a cable attachment system may subject cable to various amounts of elongation, which may eventually lead to failure. Another source of stretch to the cable comes from elastic extension of the cable under load, which is typically characterized as linear in nature. Permanent elongation can occur when high loads on the cable produce uniform plastic yielding. Additionally, localized plastic yielding may occur after a maximum breaking load is exceeded. When the cable is moved in the well, frictional forces also act on the cable and can add to the line tension especially during recovery.
Many cables have helically wound lines that generate torque when under axial load. The cables therefore tend to unlay or untwist to some extent under certain circumstances. Factors surrounding this behavior can be very difficult to predict. Even thermal expansion can occur during use of the cable, although thermal effects may not alter the mechanical properties of the cable's composition.
With all of the forms of elongation, twisting, plastic deformation, etc. that a cable can encounter, the service history of the cable needs to monitored and logged to determine what loads and actions the cable has been subjected to so an assessment can be made whether the cable is still serviceable or not. Additionally, operators need to monitor and tabulate the length of the cable to know where tools are actually located in the well and to perform various operations downhole with the cable.
Although there are many types of downhole cables known in the art and even though they may be effective, operators are continually increasing the types of uses for downhole cables and subjecting the cables to ever changing conditions and environments. To that end, the subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.